Many non-destructive testing applications use x-ray radiation to locate objects that are within other structures. For example, x-ray detection is used to image dense objects, such as human bones, that are located within the body. Another application of x-ray detection and imaging is in the field of non-destructive electronic device testing. For example, x-ray imaging is used to determine the quality of solder that is used to connect electronic devices and modules to printed circuit boards.
X-ray imaging works by passing electromagnetic energy at wavelengths of approximately 0.1 to 100×10−10 meters (m) through the target that is to be imaged. The x-rays are received by a receiver element, known as an x-ray detector, on which a shadow mask that corresponds to the objects within the target is impressed. Dark shadows correspond to dense regions in the target and light shadows correspond to less dense regions in the target.
The dark portions of the shadow mask generally correspond to the objects in the target that are denser than surrounding areas within the target. In this manner, dense objects, such as solder, which contains heavy metals such as lead, can be visually distinguished from less dense regions. This allows the solder joints to be inspected easily.
One way of using x-rays to develop an image of an object is to convert the x-ray light energy into light energy having a longer wavelength than x-rays. The longer wavelength light energy is then applied to a light capturing device, which converts the light into an electrical signal. One such device for converting x-rays into longer wavelength light energy is called a scintillator. A scintillator includes, for example, a cesium iodide strip placed on a substrate of aluminum or carbon. The x-rays impact the cesium iodide, which releases photons at a wavelength of approximately 560 nanometers (nm). The light capturing device also includes an optical path for conducting the photons emitted from the cesium iodide to a light sensitive device. The optical path can be, for example, air, and the light sensitive device can be, for example, an array of charge-coupled device (CCD) elements. The CCD elements convert the photons into an electrical signal that is directed to a signal processing device so that the electrical signals can be transformed into a usable image that depicts the dense regions in the target.
Prior methods used to couple the light from the scintillator to the CCD elements include using conventional reduction optics and focusing the image produced by the scintillator onto the CCD using conventional lenses.
Existing x-ray inspection systems require that the target be placed between the scintillator and the x-ray, and require that the scintillator be movably mounted. The source scintillator is continuously rotated and the x-ray source is steered in a direction opposite the rotation direction of the scintillator so that, at a particular height, referred to as a “z-height,” the target will be in focus. One of the drawbacks of such a system is the high cost associated with mechanically mounting and rotating the scintillator. Another drawback is that only one plane is in focus at any time. This requires that the topology of the target (i.e., a printed circuit board (PCB) including components mounted thereon) be known prior to performing the x-ray inspection. Unfortunately, topology mapping is costly, difficult and time consuming.
One manner of addressing some of these drawbacks is to use a number of discrete x-ray detectors arranged, for example, in a circular pattern. Such an arrangement allows a number of images to be captured and synthesized so that an x-ray slice can be created for more than one z-height.
One of the drawbacks encountered when converting x-rays to an electrical signal is that not all of the x-rays that impinge on the cesium iodide scintillator are converted to photons. Some of the x-rays pass through the scintillator and tend to impinge on the CCD array to a degree such that the x-rays degrade the CCD elements over time. Further, the x-rays tend to create noise in the CCD elements, thereby degrading the quality of the electrical signal supplied by the CCD array.
One manner to address this is to provide a material between the scintillator and the CCD array that can couple the photons to the CCD array, while at the same time attenuating the x-rays before they reach the CCD array. One possible solution is to use a dense glass material to couple the photons to the CCD array. Unfortunately, existing glass materials that could attenuate the x-rays would be overly thick and would tend to degrade (i.e., become yellow) when exposed to x-rays over an extended period of time.
Therefore, there is a need in the industry for a simple and efficient x-ray detector and x-ray system that will not degrade over repeated exposure to x-rays.